The present invention pertains generally to the field of photometric detection and analysis, and more particularly to apparatus and methods for detecting and/or measuring the energy absorption by a fluid medium in a sample chamber.
In the qualitative and quantitative analysis of fluids, a well known analysis technique calls for a beam of electromagnetic radiation, such as visible light or infrared radiation, to be time-chopped (on-off modulated) to form a sample and a reference beam. The sample beam is directed through a sample of a fluid medium being examined; and selected components within the medium, if present, attentuate the energy of the incident beam at particular wavelengths characteristic of the selected fluid components. By measuring the amplitude of the sample beam, relative to the reference beam, after passing through the fluid at one or more wavelengths, it is possible to both detect and identify the selected fluid components and determine the density thereof.
Generally, however, these photometric detection and analysis measurement techniques attempt to measure small changes of signal which "ride" on a relatively large quiescent or base signal. Thus, changes of for example 0.1% or less in the overall signal are not unusual in order to accurately determine the quantity of selected fluids in the sample chamber. This happens because the amount of energy absorption at a wavelength of interest is relatively small compared to the magnitude of the incident and measured signals. Therefore, in order to make these measurements extremely precise, low noise, complex measurement techniques are generally employed.
As a result, at least one prior art system mechanically modulates, by successively blocking and unblocking, a reference and a sample beam respectively, for providing two correlated modulated waveform beams having a 180.degree. out of phase AC component. These two beams, if not attenuated or if attenuated by the same amounts, can be combined to provide a single measurement signal having a constant amplitude. If however one of the beams is differentially attenuated, the combined measurement signal will have an AC component which can be measured and which is proportional to the difference in the amount of attenuation in the two amplitude modulated signals. This prior art apparatus improves the accuracy of the absorption measurement in a gas analysis system but suffers from the requirement of precision mechanical modulation of separate energy signals and the difficulties concommittant therewith.
In addition, prior art devices generally use a single interrogation wavelength during the course of an investigation. To make measurement determinations at a plurality of wavelengths, it is generally necessary to replace either the energy source or the monochromatic filter being used, and to recalibrate the system accordingly. This procedure can prove both time consuming and inefficient.
Objects of the present invention therefore are a fluid detection and/or analysis apparatus and method with improved precision, automatic operation, high efficiency, high reliable and relatively inexpensive manufacturing cost.
A further object of the present invention is a fluid detection and/or analysis apparatus and method having reduced complexity.
Yet a further object of the present invention is a fluid detection and/or analysis apparatus and method which automatically provide measurements at different wavelengths of interest.